DESCRIPTION: (Adapted from the applicant's abstract and Specific Aims.) The diaphragm is the primary respiratory muscle. Hyperexpansion of the thoracic cavity is thought to impair diaphragm function, and diaphragm fatigue is believed to be a major factor in hypercarbic respiratory failure. The mechanism by which diaphragm muscle contraction is converted into transdiaphragmatic pressure and lung expansion is poorly understood. The objective is to describe the relation between muscle contraction and diaphragm function by determining the quantitative relations among muscle tension, muscle length (L), diaphragm shape, transdiaphragmatic pressure (Pdi) and lung. Aim 1 will use the biplane video fluoroscopic technique to measure the lengths of at least four bundles, the global shape of the diaphragm, and Pdi in dogs: a) during spontaneous inspiratory efforts against an occluded airway and the release from occlusion, and b) during contractions produced by selective stimulation of the left and right phrenic nerves and left and right 5th, 6th and 7th cervical nerve roots. In Aim 2, tension-length curves of passive and active rat and canine diaphragm muscle under biaxial loading will be measured. In Aim 3, the stress distributions in the active diaphragm will be computed from the data on diaphragm shape and Pdi obtained in Aim 1. In Aim 4, the relationship between diaphragm muscle shortening and volume displaced by the diaphragm will be determined, and the relation between diaphragm volume displacement and volume measurements obtained by respitrace will be tested. Diaphragm function is impaired at high volume such as occurs in patients with airway obstruction. This study may elucidate whether this failure to generate an effective Pdi is due to maximal shortening or a shape change such that the diaphragm becomes too flat to generate pressure and actually exerts an expiratory force on the rib cage.